A Nutrient Basis for the Distribution of the Lemnaceae.

This study predicts the outcome of competitive interactions in the Lemnaceae (Duckweeds) based upon growth responses to limiting resources and relates this information to the species' natural distribution. Light does not determine inter-pond species distributions. Spirodela polyrhiza has the lowest half-saturation constant (K) for light, 24 (mu)Ein/m('2)(.)s, yet it is not found in continuously shaded ponds. However, the cooler temperatures in these ponds favor Lemna minor, which is attributed to the superior growth rate of this species below 20(DEGREES)C. Iron may be involved in the separation of the S. polyrhiza-Wolffia columbiana complex from the L. minor-L. turionifera complex. Above pH 8, where iron precipitates, the former species maintain growth while the latter are inhibited. S. polyrhiza and W. columbiana are found in the field at high pH (8-9) and low iron (0.53 (mu)M) as compared to the other species. Phosphate-limitation favors L. turionifera, which has the lowest K, 0.35 (mu)M P, of the species studied. It is found in nature at a lower concentration, 0.36 (mu)M P, than L. minor, S. polyrhiza or W. columbiana. In the laboratory L. turionifera excludes S. polyrhiza in competition at high N/P ratios. Nitrogen does not seem to affect species distributions. Although W. columbiana has a significantly lower K for nitrate, 2.2 (mu)M N, than the other species, it is never dominant in the ponds studied. This is attributed to the supply of available nitrogen in rainfall, 52 (mu)M N. L. turionifera and S. polyrhiza both have K values near 5 (mu)M N, which is why they coexist at low N/P ratios in competition experiments. W. punctata was the most inefficient species in the utilization of the three nutrients, it is excluded at all N/P ratios in competition with L. turionifera, but it is also found in ponds with low concentrations of all three nutrients. W. punctata may avoid competition by obtaining nutrients from the sediments by completing a sinking-floating cycle in as little time as a week. The other species do not have this capability.Ph.D.EcologyUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/159531/1/8324164.pd